The present invention relates to a passivated and encapsulated semiconductor and methods of making same, and more particularly to a glass passivated and plastic encapsulated semiconductor and method of making same.
It has long been known that semiconductor devices are more reliable and longer lived if their active surfaces are coated with a passivating and protecting coating. Many different substances have been proposed for use in such a coating, among them various types of alkali-free glasses, including an improved alkali-free zinc-borosilicate glass described in Morrissey U.S. Pat. No. 3,752,701 (issued Aug. 14, 1973 and assigned to General Instrument Corporation). While such a glass passivation/encapsulation layer provides passivation to the semiconductor junction, mechanical handling strength, and hermeticity, it typically suffers from one or more of the following disadvantages: (1) lacking reproducibility in external size due to the nature of the material and the techniques available for applying the same, (2) being oval shaped and thus hard to handle in customers' circuit boards and customer equipment, (3) being light transparent, and (4) being difficult to mark.
In addition to glass, the prior art suggests as passivation/encapsulation materials various plastics or resins. In such a device a varnish or silastic type material is employed to achieve passivation about the semiconductor junction, the junction passivating material then being overmolded or encapsulated with an epoxy or silicone type liquid or powder molding material. However, the plastic passivation/encapsulation layers of the prior art typically suffer from one or more of the following disadvantages: (1) the varnish or silastic junction passivation material does not completely protect the semiconductor against moisture, (2) both the junction passivation and encapsulating plastic materials deteriorate at high temperature operation, (3) the encapsulative plastic provides poor permeability protection relative to glass, so that the plastic passivated/encapsulated devices show failures under pressure cooker type tests at 15 p.s.i, and (4) the encapsulating plastic frequently does not provide fire-retardant properties (although this feature may be incorporated into the plastic mold material).
The plastic passivated/encapsulated semiconductors antedate the glass passivated/encapsulated semiconductors and, despite their above-recited disadvantages, have been produced and used in such vast quantities that organizations of manufacturers and users have established dimensional standards therefor, thus making possible the development of automatic handling machinery for testing, marking, tape-reel packaging, lead bending and trimming, and automatic insertion of the devices into printed circuit boards. While all of the above-identified reliability problems characteristic of the plastic passivated/encapsulated devices are overcome in the glass passivated/encapsulated devices, the size and beaklike shape of the glass layer varies considerably from device to device. This variability, especially in conjunction with the miniature size of the beads in the newest devices, greatly increases the problems associated with automatic handling equipment, often to the point where cost-saving automatic techniques cannot be used and manual assembly costs must be absorbed to achieve the advantages of superior reliability. Thus, despite the reliability problems manifested by the older plastic passivated/encapsulated semiconductor, they remain most popular because of their mechanical design which is so readly adaptable to automation.
While semiconductor assemblies passivated and encapsulated in a combination of plastic and glass have been described in the prior art, such assemblies have not proven to be entirely satisfactory. Such semiconductor assemblies (as described in U.S. Pat. No. 3,149,396 and U.S. Pat. No. 3,237,272) typically possess one or more of the following disadvantages: (1) they are not applicable to axial lead semiconductors, (2) they utilize a special low melting point glass which fails to afford the aforementioned advantages of glass passivation/encapsulation, and (3) they utilize a microscopic passivation layer of grown lead silicate glass which affords passivation only at low voltage levels.
Accordingly, it is an object of the present invention to provide a passivated/encapsulated semiconductor assembly which combines the advantages of glass passivation and plastic encapsulation.
It is another object to provide such an assembly having a peripheral configuration which fits into automatic equipment designed for plastic passivated/encapsulated assemblies.
It is a further object to provide such an assembly which is completely resistant to moisture and withstands humidity and hermeticity tests with negligible failures.
A further object is to provide such an assembly which operates with high reliability at room temperature and at elevated temperatures, and exhibits superior thermal cycling resistance.
Another object is to provide a simple and economical process for manufacturing such a passivated and encapsulated semiconductor assembly.